Hydraulic brake fluid pressure proportioning valve

ABSTRACT

A proportioning valve having a differential piston adapted to reduce hydraulic pressure delivered from a hydraulic pressure source for actuating rear wheel brake cylinders of a vehicle, the differential piston being exposed at one end portion thereof to hydraulic pressure for actuating front wheel brake cylinders of the vehicle and serving to transmit the hydraulic pressure from the hydraulic pressure source to the rear wheel brake without reducing that hydraulic pressure cylinders in the event the front brake system fails, and a Y-shaped sealing member for sealing the rear brake pressure and the front brake pressure acting on the differential piston thereby for providing reliability in operation and ease of manufacturing.

United States Patent [1 1 Yabuta et al. 51 May 29, 1973 [54] HYDRAULICBRAKE FLUID PRESSURE [57] ABSTRACT PROPORTIO I VALVE A hydraulic brakefluid pressure proportioning valve [75] Inventors: Keiichiro Yab ta, AhLk adapted to be interposed in a dual brake system Y k h Kaname name,between a tandem master cylinder and rear brake b h f Japan cylindersfor modulating the fluid pressure to be transmitted to the rear brakecylinders. The brake fluid [73] Assignees: Nissan Motor Company Limited,

Yokohama and Sumitomo Electric Industries, Limited, Osaka, both of JapanFiled: July 6, 1972 Appl. No.: 269,455

[30] Foreign Application Priority Data Aug. 30, 1971 Japan ..46/66404[52] US. Cl ..303/6 C, 188/152, 200/82,

303/84 A [51] Int. Cl. ..B60t 8/26 [58] Field of Search ..303/6 C, 84 A;60/545; 200/82; 188/151, 152

[56] 7 References Cited UNITED STATES PATENTS 3,448,230 6/1969 Bueler..303/6 C X 3,597,008 8/1971 Falk ..303/6 C 3,667,810 6/1972 Silagy..303/6 C Primary Examiner-Allen N. Knowles Assistant Examiner-MichaelMar Attorney-John Lezdey pressure proportioning valve includes a housinghaving first and second inlet ports communicating with front and rearbrake fluid pressure sections of the tandem master cylinder,respectively, an outlet port communicating with the rear brake cylindersand a first chamber for providing fluid communicator between the secondinlet port and the outlet port to pass an unmodulated fluid pressurefrom the second inlet port to the outlet port when a fluid pressure inthe second inlet port is lower than a predetermined point of transition.The brake fluid pressure proportioning valve also includes a pressureresponsive differential piston which is axially slidably accommodated inthe second chamber and which has an upper stern portion extending intothe second chamber, a valve head and a lower stem portion. A valvemember is operatively disposed in the first chamber and cooperative withthe valve head of the differential piston to control the fluidcommunication between the second inlet port and the outlet port therebyto modulate the fluid pressure to be delivered to the outlet port whenthe fluid pressure at the second inlet port reaches the predeterminedpoint. A sealing member is disposed in the second chamber for sealingthe second chamber from the first chamber so that a failure in the frontbrake pressure will modify the functioning of the differential piston.

9 Claims, 8 Drawing Figures HYDRAULIC BRAKE FLUID PRESSURE PROPORTIONINGVALVE This invention relates to vehicular hydraulic brake fluid systemsand more particularly to a brake fluid pressure proportioning valve forvarying the hydraulic pressure to be delivered to the rear brakes ascompared to the'hydraulic pressure to be delivered to the front brakes.

It is well known in the art that the proportion of the weight of avehicle that is born by the wheel of a given axle or a given pair ofwheels does not remain static. This is because of the fact that duringrapid deceleration a significant portion of the weight of the vehicle istransferred from the rear wheels to the front wheels by the action ofmoment of inertia resultingfrom the deceleration. The amount of theweight transfer for a given vehicle is proportionally varied inaccordance with the magnitude of the deceleration.

The rate of deceleration of the vehicle is dependent upon the weightcarried by that particular vehicle and the condition of the road surfaceon which the vehicle runs on and the rate of deceleration decreases asthe total weight of the vehicle is increased or the road surface becomesmore slippery. In addition to these factors, the rate of deceleration ofthe vehicle is further varied by other several factors.

As already described hereinabove, the amount of the weight transfer isvaried in accordance with the variation in the rate of deceleration, sothat it is difficult to actuate the front and rear brakes in a manner toproduce equal sliding tendencies for its front and rear wheels undersubstantially all road conditions and rates of deceleration. To solvethis problem, the front and rear brake cylinders of the vehicle are sosized as to apply forces to the front and rear brakes which are of thedesired ratio to provide a balanced braking effort at a given rate ofdeceleration. During extremely rapid deceleration or panic brakeapplications, however, an excess amount of braking force is applied tothe rear wheels so that the driver loses directional control over thevehicle and invites a serious danger to the vehicle occupants.

To prevent the risk of this occurring, various devices have heretoforebeen proposed which serve to proportionally reduce the hydraulicpressure to be applied to the rear brakes as compared with the hydraulicpressure to be applied to the front brakes during rapid deceleration.One typical example of such devices is a brake pressure proportioningvalve which is interposed in the fluid pressure line or circuit leadingto the rear brakes in a manner well known.

In a known motor vehicle, a simple brake system is employed which is soarranged as to deliver the hydraulic brake pressure both to the frontand rear brake cylinders concurrently through a series of fluid pressurelines from a single source of hydraulic pressure, that is, a mastercylinder. It is sometimes experienced in this prior single brake systemthat failure of one component of the brake system makes the brakesinoperable.

In order to overcome this drawback encountered in the prior art,-it hasbeen proposed and put into practice to have the brake system dividedinto at least two independent fluid circuits, so that even in the eventof one circuit failing the other circuits are still left intact. Thebrake system having such two or more independent fluid circuits iscalled a dual hydraulic brake system with one or more tandem mastercylinders. The tandem master cylinder has one section communicating withthe brake cylinders of the front brakes and a second sectioncommunicating with the fluid circuit of the rear brakes, the brakepressure proportioning valve being interposed in the fluid circuitbetween the tandem master cylinder and the rear brake cylinders. Such adual hydraulic brake system has an advantage in that, upon failure inone fluid circuit or one section of the tandem master cylinder, theother fluid circuit remains effective to slow down the vehicle.

If, now, it is desired to have the prior art pressure proportioningvalve combined with the dual brake system, a problem is encountered inwhich only a reduced fluid pressure is transmitted to the rear brakecylinders in the event a failure in the fluid lines leading to the frontbrake cylinders. When any of the fluid lines to the front brakecylinders fails for-one reason or another, only the rear brakes areresponsible for the braking, with the front brakes inoperable. Since,however, the fluid pressure to be delivered to the rear brake cylindershas been reduced by the pressure proportioning valve, the rear brakesreceive a pressure which is determined by merely for the purpose ofpreventing the rear wheels from being locked when the brake pedal isdepressed hard. The braking effort exerted by the rear brakes is, as aconsequence, insufficient for the desired deceleration of the vehicle,thus endangering the vehicle occupants.

To compensate for this disadvantage encountered in the prior art dualbrake system, various attempts have heretofore been made to the brakepressure proportioning valve with a view to providing a maximum brakingefficiency. To this end, the brake proportioning valve is so constructedas to proportionally reduce the brake pressure to be delivered to therear brake cylinders when the front and rear brake systems operatecompletely and to lose the pressure regulating function thereby to causethe fluid pressure transmitted from the master cylinder to be directlypassed to the rear brake cylinders in the event the front brake systemfails.

A prior art brake pressure proportioning valve of the type abovedescribed has a pressure responsive differential piston movable againsta spring means in a valve body or having and connected in the fluidcircuit leading to the rear brakes, this differential or stepped pistonsubdividing a cylinder bore into first and second chambers communicatingwith the master cylinder section associated with the rear brakes and therear brake cylinders, respectively. The valve body also has a thirdfluid chamber which is in communication with the master cylinder sectionassociated with the front brakes and the front brake cylinders. Thedifferential piston is provided with valve means providing fluidcommunication between first and second fluid chambers during initialbrake operation wherein the differential piston is retained againstmovement by the force of the spring means. The differential piston has afirst effective cross sectional area exposed to rear system mastercylinder pressure after closure of the valve means, a second effectivearea exposed to rear brake pressure after closure of the valve means anda third effective area exposed to front system brake pressure. Duringinitial application of the brakes, the differential piston is retainedin a position to cause the valve means to provide fluid communicationbetween the master cylinder section and the rear brake cylinders by theforce of the spring means. As the brake pressure to be carried to therear brake cylinders reaches a predetermined value,

the differential piston is moved to another position to decrease theopening condition of the valve means thereby to reduce the brakepressure to be transmitted to the rear brake cylinders, whereby agreater braking force may be achieved before skid conditions arereached. Upon failure in the front brake system, the fluid pressure isnot applied to the third effective area of the differential piston, sothat the differential piston is moved toward the position in which thevalve means is opened to directly pass the fluid pressure from themaster cylinder to the rear brake cylinders.

In order to prevent malfunctions of the pressure proportioning valve,there is provided in the pressure proportioning valve a sealing memberwhich serves to seal the third effective area of the pressureproportioning valve from its first and second effective sectional areasso that a failure in the front brake pressure will lose the pressureregulating function by the valve means. The magnitude of the pressureacting on this sealing member is varied in the event the front and rearbrake systems operated completely and in the event the front brakesystem fails, resulting in various modes of deformation of the sealingmember The greater the difference in modes of deformation of the sealingmember the greater is the frictional resistance of the sealing memberand, accordingly, the frictional wear of the sealing member isincreased. Failure of this sealing member leads to unreliability inoperation of the vehicular brake system.

It is, therefore, an object of the present invention to provide animproved brake pressure proportioning valve for proportionally reducingthe hydraulic pressure applied to the rear brakes as comparedwith thehydraulic pressure applied to the front brakes when a predetermineddeceleration is reached.

Another object of the present invention is to provide a hydraulic brakepressureproportioning valve which is adapted to be incorporated in adual brake system of a vehicle.

A still another object of the present invention is to provide ahydraulic brake pressure proportioning valve for use in a dual brake off a vehicle which valve is reliable in operation, easy to assemble andinexpensive to manufacture.

A further object of the present invention is to provide a hydraulicbrake pressure proportioning valve for use in a dual brake system of avehicle in which, even though a failure has taken place in any of thefluid circuits, the brakes of the remaining fluid circuits still remainoperable to exert sufficient braking effort thus saving the vehicleoccupants from a danger of collisions.

A still further object of the present invention is to provide ahydraulic brake pressure proportioning valve which is so constructed asto minimize the frictional wear of a sealing member disposed around adifferential piston of the brake pressure proportioning valve, wherebythe durability of the brake pressure proportioning valve is prolonged.

A yet further object of the present invention is to provide a hydraulicbrake pressure proportioning valve incorporating a novel elastomericsealing member having a specific configuration to reduce the frictionalresistance.

A yet further object of the present invention is to provide a hydraulicbrake pressure proportioning valve in which a sealing member surroundinga differential piston is subjected to a unique deformation.

In order to achieve these objects of the present invention, the presentinvention contemplates to provide a hydraulic brake pressureproportioning valve which is adapted to be interposed in a dual brakesystem between a tandem master cylinder and at least one of rear brakecylinders for the purpose of modulating the fluid pressure to bedelivered to the rear brake cylinders. The brake pressure proportioningvalve includes a housing having first and second inlet ports and anoutlet port, the first and second inlet ports being connected to thefront and rear brake sections of the tandem master cylinder,respectively, and the outlet port being connected to the rear brakecylinders. The housing also has formed therein a first chamber forproviding fluid communication between the second inlet and outlet portsto pass an unmodulated fluid pressure from the second inlet port to theoutlet port when a fluid pressure prevailing in the second inlet port islower than a predetermined level at a predetermined point of transition.A pressure responsive differential piston is axially slidablyaccommodated in the first chamber for controlling fluid communicationbetween the second inlet and outlet ports. The differential piston has avalve head on its middle peripheral wall, an upper stem portionterminating in a passage communicating with the first inlet port and alower hollow stem portion internally maintained at an atmosphericpressure. An annular valve member is operatively disposed in the chamberand is cooperable with the valve head of the differential piston forrestricting the flow of fluid from the inlet port to the outlet portduring a predetermined of fluid pressure at the outlet port. Thisannular valve member has an inner annular peripheral surface with whichthe valve head of the differential piston is sealingly engageable toblock the fluid communication between the first inlet and outlet portsthereby to reduce the fluid pressure at the outlet port. A compressionspring is disposed in the chamber for biasing the differential piston toa position in which the valve head thereof is unseated from the valvemember for permitting the second inlet port to communicate with theoutlet port. The housing is further provided with a second chamber inwhich a seal member is operatively provided for sealing the upper stemportion of the differential piston from the remaining portions includingvalve head and the lower stem portion so that a failure in front brakepressure will substantially modify the functioning of the differentialpiston. In a preferred embodiment, the seal member includes an annularO-ring portion sealing adjacent component parts from each other. Theseal member also includes flrst and second sealing flanges which areintegral with the O-ring portion. The first sealing flange has a lipportion which is inclined angularly upwardly. The second sealing flangehas a lip portion which is inclined angularly downwardly. When thesealing member is fitted in the second chamber, the lip portions ofrespective flanges of the seal member are deflected radially outwardlyslightly by the engagement of their inner peripheries with the outerperiphery of the upper stem portion of the differential piston toprovide proper sealing function. The sealing member is held in placewithin the second chamber by an annular shoulder formed in the housingbetween the first second chambers and a longitudinal extension of an endclosure member by which the second chamber is closed. Between theextension of the end closure member and the sealing member is disposed athrust washer which prevents the sealing member from being subjected toa twisting moment or torsional stress when the end closure member isscrewed into the housing. The inner diameter of the thrust washer issuch that the clearance between the inner diameter of the thrust washerand the joint portion of the sea] member is minimum to satisfactorilyprevent one of the first and second flanges of the seal member frombeing excessively radially outwardly deflected. A back-up ring isdisposed in the second chamber adjacent the lip portion of the firstflange of the seal member to prevent excessive upward deflection of thefirst flanges.

These and other features and advantages of the present invention willbecome more apparent from the following description when taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic view illustrating a general arrangement of ahydraulic brake system of dual type incorporating the hydraulic brakepressure proportioning valve according to the present invention;

FIG. 2 is a cross sectional view of the hydraulic brake pressureproportioning valve of the present invention as shown in FIG. 1;

FIG. 3 is an enlarged view partly in section and partly in elevation ofone of the parts of the structure shown in FIG. 2;

FIG. 4 is a sectional view of the structure illustrated in FIG. 3 takenalong the line 4-4;

FIGS. 5 and 6 are enlarged sectional views of the structure shown inFIG. 3 taken along the lines 5-5 and 6- 6 thereof, respectively;

FIG. 7 is an enlarged sectional view of another one of parts of thestructure shown in FIG. 2; and

FIG. 8 is a view showing a modification of the structure shown in FIG.7.

Referring now to the drawings and more particularly to FIG. 1, there isschematically shown a hydraulic brake fluid system of dual typeincorporating the brake fluid pressure proportioning valve implementingthe present invention. The brake system is shown to include a usualtandem master cylinder 10 having separate front and rear sections 10aand 10b, respectively. The front and rear sections 10a and 10b areoperated simulatneously by a brake pedal 12 to deliver brake fluidthrough fluid lines or circuits l4 and 16 to rear brake cylinders 18 and18 and through lines 20, 22 and 24 to front brake cylinders 26 and 26.Between the fluid lines 14 and 16 is interposed the brake pressureproportioning valve embodying the present invention which is generallydesignated by reference numeral 28. The brake fluid pressureproportioning valve 28 has an inlet port 30 communicating with the fluidline 14 and an outlet port 32 communicating with the fluid line 16leading to the rear brake cylinders 18 and 18. The brake pressureproportioning valve 28 also has an inlet port 34 communicating with thefluid line leading from the master cylinder 10a and outlet ports 36 and38 communicatingwith the fluid lines 22 and 24, respectively, whichcommunicate with the front brake cylinders 26 and 26, respectively. Itshould be noted that in the brake system shown in FIG. 1, the brakefluid pressure transmitted from the front section 10a of the tandemmaster cylinder 10 may be directly connected to the frontbrakescylinders by means of the fluid line 20 and the brake fluidpressure delivered from the rear brake master cylinder 10b is modulatedin the brake pressure proportioning valve 28 to a lower value when thebrake fluid pressure reaches a predetermined value.

The detail construction of the brake pressure proportioning valve of thepresent invention is illustrated in section in Hg. 2. As illustrated,the brake pressure proportioning valve 28 includes a cast metal valvebody or housing 40 having formed therein first and second chambers 42and 44 between which a cylindrical bore 45 is formed in the housing 40at its intermediate portion.

The chamber 42 provides communication between the inlet and outlet ports30 and 32, the inlet port 30 communicating with the fluid line 14leading from the rear section 10b of the master cylinder 10 and theoutlet port 32 communicating with the fluid line 16 leading to the rearbrake cylinders 18 and 18 (see FIG. 1). As shown, the inlet port 30 isthreaded so that the fluid line 14 is readily coupled to the inlet port30. Indicated at 46 is a fitting having formed therein a fluidpassageway 48 communicating with the inlet port 30 and the chamber 42.The fitting 46 also has a stem portion 50 which is tightly fitted intoan opening 52 formed in the housing 40. This fitting 46 serves tosealingly interconnect the end of the fluid line 14 to the inlet port 30of the housing 40. Similarly, the outlet port 32 is threaded so that thefluid line 16 is readily connected to the outlet port 32. A fitting 54is provided in the outlet port 32 and has formed therein a fluidpassageway 56 which provides fluid communication between the outlet port32 and the chamber 42. The fitting 54 also has a stem portion 58 whichis tightly fitted into an opening 60 formed in the housing 40. Thisfitting 54 functions to provide sealing connection between the end ofthe fluid line 16 and the outlet port 32.

It will be seen in FIG. 2 that the first chamber 42 is defined bycoaxial, consecutively arranged cylindrical bores 62, 64, 66 and 68.These cylindrical bores may be conveniently formed by drilling or boringthe housing 40 from its lower end. The lower end of the first chamber 42is closed by a closure member or threaded plug 70.

The plug 70 has first and second radially extending annular shoulders 72and 74, between which a cylindrical boss 76 is formed. The outerdiameter of the boss 76 is so selected as to be suitably fitted into acylindrical bore 78 formed in the housing 40 at its lowermost end. AnO-ring 80 is positioned between the annular shoulder 74 of the plug 70and an annular shoulder 82 formed adjacent the bore 78 of the housing 40to prevent the leakage of the fluid out of the housing 40 past the plug70. As shown, this plug 70 has a threaded portion which engages withcorresponding threaded portion formed on the cylindrical bore 62 of thehousing 40. It should be noted that the longitudinal length of thecylindrical boss 76 is determined to have a value to cause the annularshoulder 74 to assume a suitable position with respect to the positionof the annular shoulder 82 of the housing for thereby appropriatelypressing the -ring 80 after the plug 70 is screwed into the housing 40.

As seen in FIG. 2, the plug 70 is also formed with a seal housing 84 anda central blind bore 86 at its upper end, the seal housing 84 and thecentral blind bore 86 and facing the first chamber 42. The central blindbore 86 is made coaxial with the cylindrical bore 45 so that the bores86 and 45 serve to slidably support and guide a pressure responsivevalve element or differential piston 88 havingan elongated lower sternportion 90 piloted in the blind bore 86 of the plug 70. The blind bore86 has a bottom end 86a which limits the downward movement of the lowerste'm portion 90 of the differential piston 88.

The lower stem portion 90 of the differential piston 88 has a firsteffective sectional area indicated at A and is formed with an elongatedcentral blind opening or cavity 92 which is open at its lower end to theblind bore 86 of the plug 70. The cavity 92 is filled with atmosphericair, which is compressed by the downward movement of the differentialpiston 88 to reduce the compressivility of atmospheric air between theend wall of the lower stem portion of the differential piston 88 and thebottom end of the blind bore 86 of the plug 70 for thereby effectingsmooth axial reciprocatory movement of the differential piston 88. Theprovision of the cavity 92 in the lower stem portion 90 of thedifferential piston 88 is reflected by a light weight to cause smoothreciprocatory movement of the differential piston 88.

A cup-type annular seal 94 is located in the seal housing 84 formed inthe plug 70. The seal 94 is arranged so that an outer flange 94a thereofengages with the wall of the seal housing 84 and an inner flange 94bengages with the outer periphery of the lower stern portion 90 of thedifferential piston 88 to prevent the flow of fluid into the blind bore86 of the plug 70 and the cavity 92 formed in the lower stem portion 90of the differential piston 88 as a result of its 94a and 94b beingbiased outwardly. Disposed above the seal member 94 is a springretaining member 96 which prevents the seal member 94 from being movedout of the seal housing 84 and which spring retaining member 96 will bedescribed hereinafter in detail.

The differential piston 88 also has an upper stem portion 98 which isslidably received in the cylindrical bore 45 and which has a thirdeffective sectional area indicated at C. Engaged with the outerperiphery of the upper stem portion 98 is a Y-shaped seal member 100,which will be described hereinafter in detail. The length of the upperstem portion 98 of the differential piston 88 is determined so that evenwhen the differential piston 88 is moved downward until the lowermostend of the lower stern portion 90 thereof abuts against the bottom end860 of the blind bore 86 of the plug 70, the Y-shaped seal member 100sufficiently seals off to prevent the fluid from being flown into thesecond chamber 44.

The differential piston 88 is formed with a valve head 102 below theupperstem portion 98 and a neck portion 104 below the valve head 102.The outer diameter of the valve head 102 is determined to have a valuelarger than those of upper stem portion 98 and the neck portion 104,respectively. Furthermore, the outer diameter of the valve head 102 issodetermined as to be slightly larger than the inner diameter of a valvemember 106 as will be described hereinafter in'detail. This outerdiameter of the valve head 102 will be herein referred to as a secondeffective sectional area which is represented by B.

The cylindrical bore 68 forming a part of the first chamber 42 isdetermined to have the inner diameter which is slightly larger than theouter diameter of the valve head 102 thereby to provide a fluid passageto dey liver the fluid transmitted to the inlet port 30 to the outletport 32. A radially extending annular shoulder 108 is formed on thedifferential piston 88 between the upper stem portion 98 and the valvehead 102 to limit the upward movement of the differential piston 88 whenabutting against a radially extending annular shoulder 110 formed in thehousing 40 between the bores 45 and 68.

An annular portion between the valve head 102 and the neck portion 104is formed in a rounded shaper so as to provide a valve function inassociation with the valve member 106. Thus, the valve head 102functions either to permit the transmission of the fluid pressure fromthe inlet port 30 to the outlet port 32, or to close this fluid path andmodulate the pressure at the outlet port with respect to the pressure atthe inlet port 30. This is accomplished by the axial movement of thedifferential piston in response to the various fluid pressures actingthereupon.

The outer diameter of the neck portion 104 is selected to be slightlysmaller than the inner diameter of the valve member 106 so that when thevalve head 102 disengages with the valve member 106 as seen in FIG. 2this condition will be referred to as valve opening condition and thereverse condition will be referred to as valve closing condition"hereinafter, a fluid pressure path is formed by an annular spacingbetween the inner peripheral wall of the valve member 106 and the outerperipheral wall of the neck portion 104.

The differential piston 88 is further formed with a radially extendingannular flange 112 below the neck portion 104, the annular flange 112being provided with a radially extending annular shoulder 114 on oneside thereof and a radially extending annular shoulder 116 on its otherside. The annular shoulder 116 of the flange 112 serves to push up thevalve member 106 in a position in which the valve member 106 ismaintained in a spacing between the annular shoulder 116 and a radiallyextending annular shoulder 118 formed in the housing 40 between thebores 68 and 66 during valve opening condition. The outer diameter ofthe annular flange 112 is determined to be slightly smaller than thediameter of a lip portion of the valve member 106 to provide a gaptherebetween so that a fluid pressure path is formed to transmit thefluid pressure from the inlet port 30 to the outlet port 32.

As seen from FIG. 2, the differential piston 88 is biased upwardly sothat the annular shoulder 108, formed adjacent the upper end thereofjust below the upper stem portion 98, abuts against the annular shoulder110 of the housing 40. This is accomplished by means of a compressionspring 120 which engages with the annular shoulder 114 of the flange 112at its upper end and is seated against the spring retaining member 96.The spring retaining member 96 has at its lower end a cylindrical boss122, which is fitted into the seal housing 84 of the plug 70. Thisspring retaining member 96 also has a retaining groove 124 at its upperface to support the lower end of the compression spring 120, the innerdiameter of the retaining groove 124 being selected to be slightlylarger than the outer diameter of the spring 120 to support the spring120 in an appropriate position to prevent deflection of the spring 120.

The spring retaining member 96 is further provided with a central hole126 having a diameter larger than the-outer diameter of the lower stemportion 90. Since the upper end portion of compression spring 120 isretained by the flange 112 and the lower end portion and the lower outerperipherary of the compression spring 120 is retained by the retaininggroove 124, the brake pressure proportioning valve 28 is prevented frommalfunctioning because the deflection of the compression spring 120 isprecluded. The deflection of the compression spring 120 can be moreadvantageously prevented by providing a larger diameter portion (notnumbered) on the differential piston 88 just below the flange 112 toprevent greater displacement of the upper portion of the compressionspring 120. v

A detail construction of the valve member 106 is shown in FIGS. 3, 4,and 6. As shown, the valve member 106 has a lip portion 130 which, inthe free state of the valve member 106, is inclined angularly downwardlyand radially outwardly. When the valve member is fitted in the firstchamber 42, its lip portion 130 is deflected radially inwardly slightlyby the engagement of its outer periphery with the wall of the chamber42. This prevents the upward flow of fluid from the chamber 42 aroundthe lip portion 130. The outer periphery of the valve member 106, abovethe lipportion 130, is provided with a plurality of circumferentiallyspaced axially extending side ribs 132 of generally oval shape. Theoutside diameter of the side ribs 132 is selected so that when the valvemember 106 is inserted in the cylindrical bore 66, the side ribs 132contact the wall of the bore 66. The lower surface of valve member 106is provided with a plurality of circumferentially equally spacedsemispherical projections 134 projecting downwardly from the lower sideof the valve member 106. The semispherical projections 134 abut againstthe annular shoulder 116 of the radial flange 112 and serve as props tomaintain the valve member 106 in place as shown in FIG. 2. When theannular shoulder 116 of the radial flange 112 engages with thesemispherical projections 134 formed on the lower surface of the valvemember 106, a fluid path is formed by spaces 136 between the projections134 thus permitting the fluid to flow from the inlet port 30 to theoutlet port 32.

The upper end of the valve member 106 is provided with a plurality ofangularly equally spaced ribs 138 engageable with the annular shoulder118 of the housing 40 and angularly aligned with the side ribs 132 toprovide spaces 140 therebetween for the flow of fluid from the outletport 32 out between the annular shoulder 118 and the valve member 106,to the spaces between the side ribs 132 formed on the outer periphery ofthe valve member 106. By this means, fluid pressure at the outlet port32 can gain access to the outer periphery of the lip portion 130 so thatif fluid pressure at the outlet port 32 is higher than fluid pressure atthe inlet port 30 after valve closure, the outlet pressure can force thelip portion 130 radially inwardly for the reverse flow of fluid from theoutlet port 32 to the'first chamber 42 around the valve member 106. Asbest shown in FIGS. 3 and 4, the valve member 106 is formed with arounded valve seat 142 disposed at the upper end of its inner peripheralsurface 144. This valve seat 142 is engageable with the valve head 102of the differential piston 88 upon downward movement of the piston 88against the force of the compression spring 120.

As already noted hereinabove, the diameter of the inner peripheralsurface 144 of the valve member 106 is determined to be slightly largerthan the outer diameter at the neck portion 104 of the differentialpiston 88, thus providing an annular fluid path communicating with thespaces 136 between the semispherical projections 134 to permit the fluidto flow from the inlet port 30 to the outlet port 32 during the valveopening condition. Moreover, the diameter of the inner peripheralsurface of the valve member 106 is selected to have a value such thatwhen the valve head 102 of the piston 88 engages with the valve seat 142of the valve member 106, the valve seat 142 is capable of sufficientlysupporting the valve head and providing proper sealing function, andthat if the fluid pressure at the inlet port 30 is decreased after thevalve closure, the differential piston 88 is moved downward to cause thevalve head 102 thereof to increase the volume at the outlet port sideeffectively for thereby reducing the level of the fluid pressure to beapplied to the rear brake cylinders in proportion to the fluid pressureat the inlet port 30. This is satisfactorily accomplished by selectingthe diameter of the inner peripheral surface 144 of the valve member 106in such a manner that during downward movement of the lower stem portionof the differential piston 88 toward the bottom end 86a of the blindbore 86 of the plug 70, the valve head 102 of the differential piston 88engages with the inner peripheral surface 144 of the valve member 106 tokeep the valve member 106 in its sealing function.

Turning now to FIG. 2, the second chamber 44 is defined by coaxial,consecutively arranged cylindrical bores 150, 152 and 154. It will beseen that an annular shoulder 154 is formed in the housing between thecylindrical bores 45 and 156 and that an annular shoulder 158 is formedbetween the cylindrical bores and 152. As shown, theupper end of thesecond chamber.

44 is closed by a second threaded end closure member or end plug 160which is screwed into a partly threaded the cylindrical bore 154 of thehousing 40.

The end plug 160 has first and second radially extending annularshoulders 162 and 164, between which a cylindrical boss 166 is formed.The outer diameter of the cylindrical boss 166 is so selected as to besuitably fitted into a cylindrical bore 168 formed in the housing 40 atits uppermost end. An O-ring 170 is positioned between the annularshoulder 164 of the end plug 160 and an annular shoulder 172 formed inthe housing 40 adjacent the cylindrical bore 168 to prevent the leakageof fluid out of the housing 40 past the end plug 160. It should be notedthat the height of the cylindrical boss 166 is selected to have a valueto cause the annular shoulder 164 of the end plug 160 to assume asuitable position to appropriately press the O-ring 170 to the annularshoulder 172 of the housing 40 after the end plug 160 has been screwedinto the bore 154 of the housing 40.

As seen in FIG. 2, the inlet port 34 which communicates with the fluidline 20 leading to the front section cylinder 10a (see FIG. 1) isprovided in the end plug 160 at its upper end portion. The inlet port 34is threaded so that the fluid line 20 is readily coupled to the inletport 34. A fitting 174 is provided which has a fluid passageway 176communicating with the inlet port 34. The fitting 174 also has a stemportion 178 which is tightly fitted into an opening 180 formed in theend plug 160. This fitting serves to sealingly interconnect the end ofthe fluid line 20 with the inlet port 34. The outlet ports 36 and 38,which lead to the rear brake cylinders 18 and 18' respectively (see FIG.1), are provided in the housing 40 at its upper portion and communicatewith the inlet port 34 in a manner as will be described hereinafter indetail. The outlet port 36 is threaded for the reason as previouslydescribed. A fitcommunicating with the outlet port 36. This fitting 182also has a stem portion 186 which is tightly fitted into an opening 188formed in the housing 40 for the purpose as already mentioned.Similarly, the outlet port 38 is threaded for the same reason aspreviously noted. A fitting 190 has a fluid passageway 192 communicatingwith the outlet port 38 and astem portion 194 which is tightly fittedinto an opening 196 formed in the housing 40.

The end plug 160 has a longitudinally extending cylindrical portion 198having formed therein a longitudinally extending bore or cavity 200 inwhich the upper stern portion 98 of the differential piston 88 isslidably disposed for the reason as will be discussed hereinafter indetail. The longitudinal bore 200 communicates with the inlet port 34 sothat the front brake fluid pressure is applied on the upper end of theupper stem portion 98 of the piston 88. The end plug 160 also has aradially extending opening 202 which communicates with the cavity 200.The end plug 160 is further provided with an annular recess 204 on theouter periphery of the extension 198, the annular recess 204 being inalignment with the openings 188, 196 and 202 to provide fluidcommunication therebetween. The outer diameter of the longitudinalextension 198 is selected so that the extension is suitably fitted intothe bore 152 of the housing 40 to prevent the axis of the end plug 160from being deflected with respect to the longitudinal axis of thehousing 40. The length of the extension 198 of the end plug 160 isselected so that when the end plug 160 is completely screwed into thehousing 40 until the annular shoulder 162 of the end plug 160 abutsagainst the upper end surface of the housing 40, an end surface 198a ofthe extension 198 appropriately presses the seal member 100 to theradially extending annular shoulder 158 formed in the housing 40 bymeans of a thrust washer 206 which is hereinafter described in detail.lndicatedat 208 is a back-up ring which prevents possible damage to theseal member 100. This back-up ring 208 is disposed in a cylindrical bore210 coaxial with the cavity 200. The axial length and the diameter ofthe cylindrical bore 210 are selected in a manner as will be hereinafterdescribed in detail.

FIG. 7 illustrates a detail construction of the seal member 100 shown inFIG. 2. As shown, the seal member 100 includes an O-ring portion 212which functions to seal the adjacent stationary component parts fromeach other. The seal member 100 also includes upper and lower sealingflanges 214 and 216 which are integrally formed with the O-ring portion212. The upper sealing flange 214 has a lip portion 214a which isinclined angularly upwardly. Similarly, the lower sealing flange 216 hasa lip portion'216a which is inclined angularly downwardly. When the sealmember 100 is fitted in the cylindrical bore 152, the lip portions 214aand 216a are deflected radially outwardly slightly by the engagement oftheir inner peripheries with the outer periphery of the upper stemportion 98 ofthe differential piston 88. This prevents the downward flowof fluid from the cavity 200 into the first chamber 42. It will be notedthat the novel configuration of the sealing flanges 214 and 216 of theseal member 100 not only provides minimum frictional contracts but alsoallows proper distribution of the pressure load on the respective lipportions, thus eliminating premature wear-out of the seal member 100.

The O-ring portion 212 forms a main body or an outer periphery of theseal member 100, the outer periphery being formed in an annular shapelike a usual O-ring seal. When subjected to compression by the extension198 of the end plug 160, the O-ring portion 212 of the seal member 100is radially outwardly deflected until the outer peripheral wall 218thereof engages with 'the cylindrical bore 152 of the housing 40. Whensubjected to a high pressure from the cavity 200 or from the firstchamber 42, the seal member 100 functions to prevent the flow of fluidinto either the cavity 200 or the first chamber 42 by the actions theupper and lower sealing flanges 214 and 216. The outer diameter of theseal member 100 is such that when the outer periphery of the O-ringportion 212 abuts against the cylindrical bore 152 of the housing 40upon compression of the O- ring portion 212, excessive inward deflectionof the O- ring portion 212 is prevented to reduce the pressure acting onthe upper stem portion 98 of the differential piston 88.

The outer diameter of the seal member 100 is determined by taking intoconsideration the other factors to be explained hereinafter in detail.As already described, the cylindrical bores 78, 62, 64, 66, 68 and 45 inthe housing 40 are gradually increased in diameter from its intermediateportion toward the lower end portion thereof. Accordingly, thesecylindrical bores can be readily formed in usual manners such asdrilling or boring from the lower end portion of the housing 40. Forthis reason, the cylindrical bore 78 formed at the lowermost end of thehousing is made concentric with the cylindrical bore 45 formed at theintermediate portion of the housing 40 and, consequently, thecylindrical blind bore 86 of the plug may be easily maintained inconcentric relationship with respect to the cylindrical bore 45 of thehousing 40. On the contrary, the cylindrical bores 150, 152, 154 and 168of the housing 40 are gradually increased in diameter from theintermediate portion of the housing 40 toward the upward direction andthese bores may be formed by drilling or boring the housing 40 from itsuppermost end.

A problem is encountered in this instance in that it is quite difficultto make the cylindrical bore 152 concentric with the cylindrical bore 45and thus a slight eccentricity exists between these bores 152 and 45.For this reason, the cylindrical bore 152 is made eccentric slightlywith respect to the upper stem portion 98 of the differential piston 88supported by the cylindrical bore 45 and the blind bore 86 of the plug70. Under this circumstance, if the diameter of the seal member islarger than the inner diameter of the cylindrical bore 152, the O-ringportion 212 of the seal member 100 is compressed radially inwardly toincrease the pressures of the upper and lower flanges 214 and 216 actingon the outer periphery of the upper stem portion 98 of the differentialpiston 88.'Since, moreover, the cylindrical bore 152 of the housing 40is slightly eccentric with respect to the upper stern portion 98 of thedifferential piston 88, the pressure load can not be uniformlydistributed on the respective lip portions of the upper and lowerflanges 214 and 216 of the seal member 100. For these reasons, the outerdiameter of the seal member 100 is selected to have a value slightlyless than the inner diameter of the cylindrical bore 152 of the housing40 to compensate for the possible slight degree of eccentricity betweenthe bore 152 and the upper stem portion 98 of the differential piston88.

When the level of the fluid pressure below the seal member 100 is lessthan that of the fluid pressure above the seal member 100, the upper andlower flanges 214 and 216 are biased downwardly because of the pressuredifference across the seal member 100. As this pressure differenceincreases excessively, the seal member 100 is damaged resulting in poorsealing function. Thus, the axial length and the diameter of thecylindrical bore 150 of the housing 40 are determined to have values toprevent the above-mentioned possible damages to the seal member 100 in amanner as will be described in detail. Namely, the diameter of the bore150 is so selected that the clearance 220 between the bore 150 and thejoint portion of the seal member 100 should be as small as possible.Similarly, the axial length of the bore 150 is so selected that theclearance 222 between the bottom surface of the lower flange 216 of theseal member 100 and the annular shoulder 156 of the housing should be assmall as possible. i

The O-ring portion 212 of the seal member 100 is compressed toward theannular shoulder 158 of the housing 40 by means of the extension 198 ofthe end plug 160 thereby providing sealing function. If the uppersurface of the O-ring portion 212 of the seal member 100 directlyengages with the end surface 1980 of the extension 198 of the end plug160, the O- ring portion 212 of the seal member 100 is subjected to anexcessive torsional stress when the end plug 160 is screwed into thehousing 40, thus resulting in a loss of proper sealing function ordamages to the O-ring portion 212. To solve this problem, the thrustwasher 206 is disposed between the extension 198 of the end plug 160 andthe seal member 100 as already discussed hereinabove. This thrust washer206 may be made from a suitable synthetic resin having low frictionalresistance such as a fluorine containing resin so that the O- ringportion 212 of the seal member 100 is prevented from being subjected toa twisting moment by the extension 198 of the end plug 160.

As seen from FIGS. 2 and 7, the thrust washer 206 has a cylindrical bore206a which is in alignment with the cylindrical bore 150 of the housing40 for receiving the upper flange 214 of the seal member 100. Whensubjected to a high pressure from the first chamber 42, the upper andlower flanges 214 and 2l6.of the seal member 100 are biased upwardly sothat these flanges are deflected upwardly. As the flanges 214 and 216are deflected excessively, the seal member 100 is damaged resulting in aloss of scaling function. The diameter of the cylindrical bore 206a ofthe thrust washer 206 is, therefore, so selected that the clear-ance 224between the bore 206a and the joint portion of the seal member 100 is assmall as possible to satisfactorily preventing the upper flange 214 frombeing radially outwardly deflected.

As clearly seen in FIG. 7, the back-up ring 208 is provided which servesto satisfactorily prevent the lip portion 214a of the upper flange 214of the seal member 100 from being damaged. The inner diameter of theback-up ring 208 is slightly larger than the outer diameter of the upperstem portion 98 of the differential pis ton 88 so that the back-up ring208 is slidable on the upper stem portion 98 of the piston 88. The outerdiameter of the back-up ring 208 is so selected to provide a clearance226 between the outer periphery at the upper stem portion 98 at thepiston 88 and the cylindrical bore 210 formed in the end plug 160. Thethickness of the back-up ring 208 is so selected as to have a value toprovide a clearance 228 between the upper surface of the back-up ring208 and an annular shoulder 230 formed in the end plug 160 at its lowerportion adjacent the cylindrical bore 210 for a reason to be discussedin detail.

The annular shoulder 230 of the end plug 160 functions to prevent theexcessive upward deflection of the upper flange 214 of the seal memberin association with the back-up ring 208.

The cylindrical bore or cavity 200 formed in the end plug has asufficiently large diameter to permit the upper stem portion 98 of thedifferential piston 88 to freely move therethrough, so that the upperstem portion 98 is smoothly axially movable in the cavity 200 withoutcontacting the inner peripheral wall thereof even whenthe eccentricityexists between the cavity 200 and the upper stem portion 98 of thepiston 88. The back-up ring 208 is slidably movable on the upper stemportion 98 of the piston 88 even when the eccentricity exists betweenthe cylindrical bore 210 of the end plug 160 and the upper stem portion98 of the piston 88 due to the sufficient clearance 226 between theback-up ring 208 and the cylindrical bore 210 of the end plug 160.

It will thus be seen that the back-up ring 208 serves to preventpossible damages to the lip portion 214a of the upper flange 214 of theseal member 100 without causing difficulty in smooth movement of theupper stem portion 98 of the piston 88. It will also be noted that thelip portions 214a and 216a of the respective upper and lower flanges 214and 216 of the seal member 100 are prevented from excessive deflectionand from possible damages even when a high degree of pressure differenceexists across the seal member 100 since the clearances 222 and 228 areheld to proper values. The tests have shown that the seal member 100forming an essential part of the present invention does not lose theproper sealing function even when the differential piston 88 is vibratedfor a long period of time at a speed of 200 cycles per second duringregulating condition of the fluid pressure to be applied to the rearbrake cylinders.

A modified form of the structure shown in FIG. 7 is illustrated in Flg.8, wherein like component parts are designated by the same referencenumerals as those used in FIG. 7. In this modification, the back-up ring208 is dispenced with and in place thereof, the thickness of the thrustwasher 206 is so selected that the clearance 232 between the end surface198a of the extension 198 of the end plug 160 and the upper flange 214of the seal member 100 is minimum thereby to prevent excessive upwarddeflection of the upper and lower flanges 214 and 216 of the seal member100 that would otherwise occur when the fluid pressure above the sealmember 100 becomes lower than the fluid pressure in the first chamber42. As previously mentioned, first and second clearances 234 and 236between the cylindrical bore 200 and the outer periphery of the upperstem portion 98 of the piston 88 have different values due to theeccentricity therebetween even though the inner diameter of the cavity200 is selected to have a value to provide proper clearance between thebore 200 and the outer periphery of the stem portion 98 of the piston88. If the first clearance 234 becomes extremely small, the upper stemportion 98 of the piston 88 is caused to contact the inner periphery ofthe bore 200 so that the upper stern portion 98 of the piston 88 isdamaged. Furthermore, as the first clearance 234 decreases, the secondclearance 236 increases so that the lip portion 214a of the upper flange214 of the seal member 100 is caused to be extruded into the secondclearance 236 in the event the seal member 100 is subjected to a highpressure in upward direction. On the contrary, if the upper and lowerflanges 214 and 216 of the seal member 100 are biased downwardly due toa high pressure acting in the downward direction, the lip portion 214aof the upper flange 214 is pulled out of the second clearance 236. Whenthis phenomenon repeatedly takes place, the lip portion 214a of theupper flange 214 of the seal member 100 is damaged to lose the sealingfunction. This drawback can be overcome by providing a back-up ring asstated hereinabove.

When, now, the brake pedal 12 is depressed with all the fluid circuitsnormally operable, an increased fluid pressure which may be representedby P,,,,, occurs in the inlet ports 30 and 34 leading from the rear andfront sections 10b and 10a, respectively. The fluid pressure transmittedto the inlet port 30 is delivered through the chamber 42 of the brakepressure proportioning valve 28 to the outlet 32, from which the fluidpressure is applied through the fluid circuit 16 to the rear brakecylinders 18 and 18. The fluid pressure transmitted to the inlet port 34is passed through the opening 202 of the end plug 160 of the brakepressure proportioning valve 28 to the two outlets ports 36 and 38, fromwhich the fluid pressure is applied through the fluid circuits 22 and 24to the front brake cylinder 26 and 26'. Until the fluid pressure reachesa level which is predetermined by the compression of the spring 120, thedifferential piston 88 is held in its protruded position with theannular shoulder 108 of the valve head 102 in abutting engagement withthe annular shoulder 110 of the housing 40 to cause the valve head 102to be unseated from the valve seat 142 of the valve member 106, thuspermitting the fluid pressure in the inlet port 30 to be transmitted tothe outlet port 32. As the brake pedal 12 is further depressed and thefluid pressure reaches a predetermined level which may be represented byP,,,,, then the differential piston 88 is moved toward the plug 70against the force S of the compression spring 120, whereby the valvehead 102 engages with the valve seat 142 of the valve member 106. Thenthe fluid communication between the inlet port 30 and the outlet port 32is blocked so that the fluid pressure at the outlet port 32 no longerrises irrespective of an increase in the fluid pressure P,,,,,.

Now assuming that the fluid pressures developed in the front and rearsections 10a and 10b are equal to each other, A represents the effectivesectional area of the lower stem portion 90 of the differential piston88, C represents the effective sectional area of the upper stem portion98 of the piston 88, and the fluid pressure pressure P,,,,,, thedifferential piston 88 is biased downwardly with a force P,,,,,'CP,,,,,(A C). However, this downward force exerted on the differentialpiston 88 is lower than the force S of the compression spring 120 sothat the following relationship holds:

P,,,,,-A s (1) P,,,,, is lower than the predetermined level of the fluidIn this condition, the differential piston 88 is maintained in itsuppermost position with its valve head 102 opening the valve member 106so that the fluid pressure at the inlet port 30 is delivered to theoutlet port 32.

As the brake pedal 12 is further depressed and the input fluid pressureP,,,,, reaches a predetermined level which is represented by P,,,,, thenthe differential piston 88 is biased downwardly with a force P 'CP,,,,,(A C). In this instance, the differential piston 88 is opposed bythe force S of the compression spring 120. Since, in this condition, theforce S of the compression spring 120 is lower than the downward force,the following relation exists:

M S (2) When this occurs, the differential piston 88 is moved downwardlyfor a distance A x so that the valve head 102 thereof engages with thevalve seat 142 of the valve member 106 to block the fluid communicationbetween the inlet port 30 and the outlet port 32. At this time, thedifferential piston 88 is biased downwardly with a force P,,,,,-C P, ,(BC). Where B represents the effective sectional area of the valve head102, and P represents the fluid pressure at the outlet port 32 when thefluid pressure P,,,,, reaches the predetermined level P At the sametime, the differential piston 88 is biased upwardly with an opposingforce P,,,,,(B A) S,,,, where S,,, represents the force of thecompression spring 120 where the spring 120 is compressed for thedistance A x (S, S B-A x, where ,8 represents the spring constant of thecompression spring 120). Thus, when the fluid communication between theinlet port 30 and the outlet port 32 is blocked, the differential piston88 is held in a balanced position in which ms rs( w hence P": ms wSince, in this condition, the fluid communication between the inlet port30 and the outlet port 32 is blocked when the fluid pressure P at theoutlet port 32 is equal to the predetermined level P,,,,, that is, whenP P,,,,, the fluid pressure P,,,,, is therefore written from Eq. (3)

m w/ (4) From this Eq. (4) it is apparent that the predetermined levelof the fluid pressure may be easily obtained by suitably determining theforce S of the compression spring 120 and the diameter of the lower stemportion of the differential piston 88.

After the valve head 102 closes the valve member 106 to block the fluidcommunication between the inlet and outlet ports 30 and 32 and the fluidpressure at the inlet port 30 is further increased by a level AP, by theaction of the master cylinder 10, the increased fluid pressure will acton the differential piston 88 over its effective sectional area B lessthe area A. At this instant, the differential piston 88 is biaseddownwardly with a force (P AP,,,)'C P,,(B C). The piston 88 is alsobiased upwardly with a force (P AP,,,)(B A) S,,,. Here, the upward forceis larger than the downward force. Thus, (P +AP,,,)-C P,,,(B C) (P,,,,+AP,,,)(B A) S,,,. In this condition, the differential piston 88 is movedupwardly to reopen the valve member 106 to deliver the increased fluidpressure AP to the outlet port 32. It will be noted, however, that anyof this increased fluid pressure AP delivered to the outlet port 32creates an opposing downward force on the differential piston 88 actingdownwardly on the piston 88 over the effective sectional area B. This,of

' course, tends to cause the valve head 102 of the piston 88 to reclosethe valve member 106. The condition for equilibrium of the differentialpiston 88 in this condition is expressed by:

From this and Eq. (3), the increase in fluid pressure AP,. at the outletport 32 is obtained as AP, AP -[B From this it is apparent that theratio of the fluid pressure is determined by the relationship of theeffective sectional areas A, B and C of the differential piston 88.

When the supplied fluid pressure P is equal to or greater than thepredetermined level P,,,,, that is, when P,,,,, P,,,,,, the fluidpressures P and P, at the inlet port 30 and the outlet port 32,respectively, are expressed by m ms 1m and r P, +AP,.

From these and'Eq. the following equation is obtained:

P -C P,(B C) P,,,(B A) 8,, so that From Eq. (7) it will be seen that thefluid communication between the inlet and outlet ports 30 and 32 can.

(P AP,,,)C+ P,(B C). At the same time, the piston 88 is also biasedupwardly with an opposing force (P A P,,,) (B A) S,,,. Thus, thefollowing relationship holds:

mm)' r( m m)( w From the foregoing it will be apparent that thedifferen; tial piston 88 is further moved downwardly in the condition aspreviously noted.

An important feature of the brake pressure proportioning valveimplementing the present invention resides in the operation of thedifferential piston and the valve member associated therewith. When thefluid pressure at the inlet port 30 ,is reduced, the force tending tomove the differential piston 88 upwardly are reduced and the piston 88moves downwardly under the influence of the fluid pressure at the outletport 32 as seen from Eq. (9). In this condition, the valve member 106 isstrongly pressed against the annular shoulder 118 of the housing 40between the bores 66 and 68. Under these circumstances, as thedifferential piston 88 moves downwardly, its valve head .102 slideswithin the inner peripheral surface 144 of the valve member 106, therebyincreasing the availablevolume for thefluid at the outlet port 32 andtherebyaccomplishinga reduction in pressure P,. Due tothis downwardmovement of the differential piston 88, the force of the compressionspring 120 is varied from a value 8,, to a value of S S and, therefore,the differential piston 88 is maintained in its balanced condition. Thecondition for equilibrium of the differential piston is expressed by(P,,, AP,,,)(B A) (S,,, AS) Substitution of Eq. (8) into Eq. (9) resultsin From this and Eq. (6) it is apparent that the level of the fluidpressure at the outlet port 32 obtained'when the supplied fluid pressureat the inlet port 30 increases beyond the predetermined level is smallerthan that obtained when the supplied fluid pressure decreases to thepredetermined level. It should, however, be noted that such a differencein rates of variations of the output fluid pressures is within a limitedvalue as seen from Eq. (11) and, accordingly, the brake pressureproportioning valve with operate reliably.

The brake pressure proportioning valve of the present invention also hasanother important feature in that the valve member 106 serves asa checkvalve as will be described in detail. When the fluid pressure at theinlet port 30 is maintained at a level beyond the predetermined level,the fluid pressuree at the outlet port 32 can never be greater than thefluid pressure at the inlet port 30. This tends to cause the lip portionof the valve member 106 to be pressed against the inner wall of thefirst chamber 42. Consequently, the valve member 106 functions as acheck valve to prevent the reverse flow of the fluid from the inlet port30 to the outlet port 32 through between the lip portion of the valvemember 106 and the inner wall of the first chamber 42.

It should be noted, however, that when the fluid pressure at the inletport 30 reaches a level lower than the predetermined level, that is,when P P, (P, P AP), the lip portion of the valve member 106 isdeflected radially inwardly under the influence of the pressure Pthereby to provide a fluid path for the flow of fluid from the inletport 30 to the outlet port 32 so as to transmit the pressure P thereto.Thus, the fluid pressures at the inlet and outlet ports 30 and 32 becomeequal. If we assume the fluid pressure in this condition is P then theforces exerted on the differential piston 88 are obtained from Eqs. (l)and (10) as follows:

mM w S, so that the piston 88 is biased upwardly with a force (8,, S) Sthereby to reopen the annular passage between the valve head 102 and thevalve member 106.

in the event that a failure or break takes place in the fluid circuit tothe front brake unit, then the fluid pressure at the inlet port 34reaches a zero level. In this condition, the force tending to move thedifferential piston 88 downwardly is expressed as:

P X C O From this and Eq. (7 the condition for equilibrium of thedifferential piston 88 in this condition is expressed as:

A m( w It follows that r m 10 It will now beappreciated that Eq. (13)represents the relationship between the input and output fluid pressureswhere the failure takes place in the fluid circuit to the front brakeunit. It is thus seen that the fluid pressure at the outlet port 32 isreduced at a rate of (B A)/(B C). Before the pressure proportioningvalve commences to proportionally reduce the fluid pressure to beobtained at the outlet port 32, the fluid pressure, at the inlet port 30is equal to the fluid pressure at the outlet port 32 and, therefore,P,,, P From this and Eq. (12), the relationship holds:

mr rs w If the rates of decrease in the fluid pressure at the outletport 32 are compared by using Eqs. (8) and (13 there holds relationship:

From Eqs. (4) and (13 there holds the relationship:

From the foregoing it will be apparent that upon failure in the frontbrake hydraulic pressure system the differential piston 88 of the brakepressure proportioning valve is maintained in its uppermost positionirrespective of the fluid pressure at the inlet port 30 to keep theinlet port 30 in communication with the outlet port 32 and, accordingly,the brake hydraulic pressure proportioning valve under this condition nolonger acts as a valve in its usual sense but as a mere passageway ofpass the fluid pressure freely from the inlet port 30 to the outlet port32. Thus, according to a preferred embodiment of the present invention,the brake hydraulic pressure proportioning valve becomes inoperative inthe event a failure in, for instance, the front brake pressure system sothat the fluid pressure existing at the rear brake master cylinder istransmitted as it is to the rear brake cylinders to provide asatisfactory braking effort.

It will also be noted that the brake fluid pressure proportioning valveimplementing the present invention is provided with a novel sealingstructure having a long life and proper scaling function and thus thevalve is highly reliable in operation.

What is claimed is:

1. In a dual hydraulic brake system having front and rear brakecylinders and a tandem cylinder having front and rear sectionsrespectively connected through two independent fluid circuits with frontand rear brake cylinders for separately actuating said front and rearbrake cylinders, a brake pressure proportioning valve interposed betweenthe tandem cylinder and the rear brake cylinders, said brake pressureproportioning valve comprising: a housing having a first inlet portcommunicating with said front section of said tandem cylinder and saidfront brake cylinders for providing constant fluid communicationtherebetween, a second inlet port communicating with said rear sectionof said tandem master cylinder, and an outlet port communicating withsaid rear brake cylinders; first and second chambers formed in saidhousing, said first chamber providing fluid communication between saidsecond inlet port and said outlet port to pass an unmodulated fluidpressure from said second inlet port to said outlet port when a fluidpressure prevailing in said second inlet port is lower than apredetermined level, and said second chamber communicating with saidfirst inlet port; a pressure responsive differential piston comprising afirst portion exposed to a fluid pressure in said second chamber and asecond portion exposed to a fluid pressure in said first chamber, saiddifferential piston being responsive to normal changes in said fluidpressure in said first cham-- ber for transmitting a modulated fluidpressure from said second inlet port to said outlet port when said fluidpressure in said second chamber exceeds said predetermined level andresponsive to a failure in front brake fluid pressure for increasingsaid modulated pressure; a valve member operatively disposed in saidfirst chamber and being cooperative with said second portion of saiddifferential piston for restricting the flow of said fluidpressure fromsaid second inlet port to said outlet port when said, fluid pressure insaid first chamber increases beyond said predetermined level; and asealing member operatively disposed in said second chamber for sealingsaid second chamber from said first chamber thereby to modify theoperation of said differential piston upon said failure in said frontbrake fluid pressure said sealing member including an O-ring portionhaving an outer periphery sealingly engageable with the wall of saidsecond chamber and upper and lower flanges integral with said O-ringportion, said upper flange being inclined angularly upwardly, said lowerflange being inclined angularly downwardly, and said upper and lowerflanges sealingly engaging with the outer periphery of said firstportion of said differential piston exposed to said fluid pressure insaid second chamber.

2. In a dual hydraulic brake system having front and rear brakecylinders and a tandem cylinder having front and rearsectionsrespectively connected through two independent fluid circuitswith said front and rear brake cylinders for separately actuating saidfront and rear brake cylinders, a brake fluid pressure proportioningvalve interposed between said tandem cylinder and said rear brakecylinders, said brake pressure proportioning valve comprising: a housinghaving a rear brake pressure inlet port communicating with said rearsection of said tandem cylinder, and an outlet port communicating withsaid rear brake cylinders; first and second chambers formed in saidhousing, said first chamber providing fluid communication between saidrear brake fluid pressure inlet port and said outlet port to pass anunmodulated fluid pressure from said second inlet port to said outletport when a fluid pressure prevailing in said first chamber exceeds apredetermined level; an end closure member screwed into said housing forclosing said second chamber and having a front brake fluid pressureinlet port communicating with said front section of said tandem cylinderand said front brake cylinders, said end closure member also having alongitudinal cylindrical extension disposed in said second chamber, saidcylindrical extension having formed therein a cylindrical borecommunicating with said front brake fluid pressure inlet port; apressure responsive differential piston comprising first portionextending into said cylindrical cavity of said extension of said endclosure member and exposed to a fluid pressure therein and a secondportion received in said first chamber and exposed to a fluid pressuretherein, said differential piston being responsive to normal changes insaid fluid pressure in said first chamber for transmitting a modulatedfluid pressure from said rear brake fluid pressure inlet port to saidoutlet port when said fluid pressure prevailing in said first chamberexceeds a predetermined level and responsive to a failure in front brakefluid pressure for increasing said modulated fluid pressure; a valvemember operatively disposed in said first chamber, said valve memberbeing cooperative with said second portion of said differential pistonfor restricting the flow of said fluid pressure from said rear brakefluid pressure inlet port to said outlet port when said fluid pressurein said first chamber increases beyond said predetermined level; and asealing member operatively disposed in said second chamber for sealingsaidsecond' chamber from said first chamber thereby to modify theoperation of said differential piston upon said failure in said frontbrake fluid pressure, said sealing member including an O-ring portionhaving an outer periphery sealingly engageable with the wall of saidsecond chamber and upper and lower flanges integral with said O-ringportion, said upper flange being inclined angularly upwardly, said lowerflange being inclined angularly downwardly, and said upper and lowerflanges sealingly'engaging with the outer periphery of said firstportion of said differential piston exposed to said fluid pressure insaid cylindrical cavity formed in said cylindrical extension of said endclosure member.

3. In a dual hydraulic brake system as claimed in claim 2, wherein saidhousing of said brake fluid pressure proportioning valve has a radiallyextending annular shoulder on the lower end portion of said secondchamber, and wherein said sealing member is interposed between saidextension of said end closure member and said annular shoulder.

4. In a dual hydraulic brake system as claimed in claim 3, wherein saidbrake fluid pressure proportioning valve further comprises a thrustwasher which is disposed between said extension of said end closuremember and said sealing member.

5. In a dual hydraulic brake system as claimed in claim 4, wherein saidthrust washer is made from a material having'a low frictionalresistance.

6. In a dual hydraulic brake system as claimed in claim 4, wherein theinner diameter of said thrust washer is such that the clearance betweenthe inner diameter of said thrust washer and the joint portion of saidsealing member is minimum to prevent said upper flange of said sealingmember from being excessively deflected radially outwardly.

7. In a dual hydraulic brake system as claimed in claim 6, wherein saidhousing of said brake fluid pressure proportioning valve has acylindrical bore adjacent said annular shoulder into which said lowerflange of said sealing member extends, the inner diameter of saidcylindrical bore being such that the clearance between the innerdiameter of said cylindrical bore and said joint portion of said sealingmember is minimum to prevent said lower flange of said sealing memberfrom being excessively deflected radially outwardly.

8. In a dual hydraulic brake system as claimed in claim 4, wherein saidbrake fluid pressure proportioning valve further comprises a back-upincomplete ring which is received in an annular recess formed in thelowermost end of said extension and slidably movable on said firstportion of said differential piston, and said back-up ring engaging saidupper flange of said sealing member for preventing a lip portion of saidupper flange from being damaged.

9. In a hydraulic brake fluid pressure proportioning valve for use in adual hydraulic brake system of a vehicle including front and rear brakecylinders and a tandem cylinder having front and rear sectionsrespectively connected through two independent fluid circuits with .saidfront and rear brake cylinders for separately actuating said front andrear brake cylinders, said hydraulic brake fluid pressure proportioningvalve being adapted to be interposed between said tandem cylinder andsaid rear brake cylinders, and said hydraulic brake fluid pressure inletport communicating with said rear section of said tandem cylinder and anoutlet port communicating with said rear brake cylinders, first andsecond chambers formed in said housing, said first chamber providingfluid communication between said rear brake fluid pressure inlet portand said outlet port to pass an unmodulated fluid pressure from saidrear brake fluid pressure inlet port to said outlet port when a fluidpressure prevailing in said first chamber exceeds a predetermined level,an end closure member screwed into said housing for closing said secondchamber and having a front brake fluid pressure inlet port communicatingwith said front section of said tandem cylinder, said end closure memberhaving a longitudinal extension disposed in said second chamber, saidcylindrical extension having formed therein a cylindrical bore, saidcylindrical bore communicating with said front brake fluid pressureinlet port, a pressure responsive differential piston responsive tonormal changes in a fluid pressure in said first chamber fortransmitting a modulated fluid pressure from said rear brake fluidpressure inlet port to said outlet port when said fluid pressure in saidfirst chamber exceeds said predetermined level and responsive to afailure in front brake fluid pressure for increasing said modulatedfluid pressure to be delivered to said outlet port, said differentialpiston having a first effective sectional area exposed to a fluidpressure in said rear brake pressure inlet port, a second effectivesectional area exposed to a fluid pressure in said outlet port and athird effective sectional area exposed to a fluid pressure in said frontbrake fluid pressure inlet port, a valve member operatively disposed insaid first chamber and cooperative with said second sectional area ofsaid differential piston for restricting the flow of said fluid pressurefrom said rear brake pressure inlet port to said outlet port when saidfluid pressure in said first chamber exceeds said predetermined level,and a compression spring disposed in said first chamber for urging saiddifferential piston to a position in which said valve head is unseatedfrom said valve member for permitting said rear brake pressure inletport to communicate with said outlet port, the improvement comprising asealing member which is operatively disposed in said second chamber forsealing said third effective sectional area from said first and secondsectional areas thereby to modify the operation of said differentialpiston so as to increase said modulated fluid pressure to be deliveredto said outlet port upon said failure in said front brake pressure, saidsealing member including an annular O-ring portion which is interposedbetween an annular shoulder formed in said second chamber of saidhousing and said cylindrical extension of said end closure member, anupper flange integral with said O- ring portion and having a lip portionangularly inclined angularly upwardly and a lower flange portionintegral with said O-ring portion and having a lip portion angularlyinclined angularly downwardly, said lip portions of said upper and lowerflanges'sealingly engaging said third effective sectional area of saiddifferential piston. =0: :0:

1. In a separated hydraulic brake system having front and rear brakecylinders and a master cylinder having front and rear sectionsrespectively connected through independent fluid circuits with saidfront and rear brake cylinders, a brake pressure proportioning valveinterposed between the rear section of said master cylinder and saidrear brake cylinders, said brake pressure proportioning valvecomprising: a housing having a first inlet port communicating with saidfront section of said master cylinder, a second inlet port communicatingwith said rear section of said master cylinder, and an outlet portcommunicating with said rear brake cylinders; first and second chambersformed in said housing, said second chamber providing fluidcommunication between said second inlet port and said outlet port so asto pass an unmodulated fluid pressure from said second inlet port tosaid outlet port when a fluid pressure prevailing in said second inletport is lower than a predetermined level, and said first chambercommunicating with said first inlet port to receive fluid pressure froma front brake pressure circuit; a pressure responsive differentialpiston disposed in said first and second chambers and having an upperstem portion exposed to a fluid pressure in said first chamber, a lowerstem portion exposed to a fluid pressure in said second chamber and avalve head formed between said upper and lower stem portions, saidpressure responsive differential piston serving to modulate the fluidpressure admitted to said second fluid chamber at a predetermined ratewhen the fluid pressure in said first and second fluid chambers exceedssaid predetermined level under a circumstance in which a braking fluidcircuit is held in its normal condition for transmitting a modulatedfluid pressure to said outlet port and serving to modify said rate ofmodulating the fluid pressure in response to a failure in the frontbrake fluid pressure circuit for transmitting a fluid pressure which ishigher than said modulated fluid pressure to said outlet port; a valvemember operatively disposed in said second chamber of said housing andbeing cooperative with said valve head of said pressure responsivedifferential piston for restricting the flow of said fluid pressure fromsaid second inlet port to said outlet port when said fluid pressure insaid first and second chambers increase beyond said predetermined level;and a sealing member operatively disposed in said housing between saidfirst and second chambers and slidably accommodating therein said upperstem portion of said pressure responsive differential piston forproviding sealing function between said first and second chambers, saidsealing member including an O-ring portion having an outer peripherysealingly engageable with the wall of said first chamber and upper andlower flanges integrally formed with said O-ring portion at its innerperiphery, said upper and lower flanges having lip portions respectivelywhich are slightly smaller in diameter before insertion of said upperstem portion of said pressure responsive differential piston and saidupper flange being inclined angularly upwardly toward the axis of saidpressure responsive differential piston for thereby sealing off saidfirst chamber and said lower flange being inclined angularly downwardlytoward the axis of said pressure responsive differential piston forthereby sealing off said second chamber.
 2. In a separated hydraulicbrake system having front and rear brake cylinders and a master cylinderhaving front and rear sections respectively connected throughindependent fluid circuits with said front and rear brake cylinders, abrake pressure proportioning valve interposed between the rear sectionof said master cylinder and said rear brake cylinders, said brakepressure proportioning valve comprising; a housing having a first inletport communicating with said front section of said master cylinder, asecond inlet port communicating with said rear section of said mastercylinder, and an outlet port communicating with said rear brakecylinder; first and second chambers formed in said housing, said secondchamber providing fluid communication between said second inlet port andsaid outlet port to pass an ummodulated fluid pressure from said secondinlet port to said outlet port until a fluid pressure prevailing in saidsecond inlet port exceeds a predetermined level; a first end closuremember screwed into one end of said housing for closing an open end ofsaid first chamber and including a longitudinal cylindrical extensionhaving formed at its one end an opening, said longitudinal cylindricalextension also having a cylindrical cavity provided with a fluidpassageway communicating with said first inlet port; a second endclosure member screwed into another end of said housing for closing anopen end of said second chamber; a pressure responsive differentialpiston disposed in said first and second chambers and having an upperstem portion slidably accommodated in said cylindrical cavity of saidfirst end closure member, said upper stem portion being exposed to afront brake fluid pressUre in said first chamber, a lower stem portionexposed to a fluid pressure in said second chamber and a valve headformed between said upper and lower stem portions, said pressureresponsive differential piston serving to modulate the fluid pressureadmitted to said second fluid chamber at a predetermined rate when thefluid pressure in said first and second chambers exceeds saidpredetermined level under circumstance in which a braking fluid circuitis held in its normal condition for transmitting a modulated fluidpressure in said outlet port and serving to modify said rate ofmodulating the fluid pressure in response to a failure in the frontbrake fluid pressure circuit for transmitting a fluid pressure which ishigher than said modulated fluid pressure to said outlet port; a valvemember operatively disposed in said second chamber of said housing andbeing cooperative with said valve head of said pressure responsivedifferential piston for restricting the flow of said fluid pressure fromsaid second inlet port to said outlet port when said fluid pressure insaid first and second chambers increase beyond said predetermined level;and a sealing member operatively disposed in said housing between saidfirst and second chambers and slidably accommodating said upper stemportion of said pressure responsive differential piston for providingsealing function between said first and second chambers, said sealingmember including an O-ring portion formed at an outer periphery thereofsealingly engageable with the wall of said first chamber and upper andlower flanges integrally formed with said O-ring portion at an innerperiphery of said sealing member, said upper and lower flanges havinglip portions respectively which are slightly smaller in diameter beforeinsertion of said upper stem portion of said pressure responsivedifferential piston, and said upper flange being inclined angularlyupwardly toward the axis of said pressure responsive differential pistonfor thereby sealing off said first chamber and said lower flange beinginclined angularly downwardly toward the axis of said pressureresponsive differential piston for thereby sealing off said secondchamber.
 3. A brake pressure proportioning valve according to claim 2,wherein said housing has a radially extending annular shoulder formed atthe lower end portion of said first chamber, and wherein said O-ringportion formed at the outer periphery of said sealing member issealingly interposed in a cavity formed between an end portion of saidcylindrical extension of said first end closure member and said annularshoulder.
 4. A brake pressure proportioning valve according to claim 3,wherein said sealing member is sealingly interposed in said cavityformed between the end portion of said cylindrical extension of saidfirst end closure member and said annular shoulder by means of a thrustwasher.
 5. A brake pressure proportioning valve according to claim 4,wherein said thrust washer is made from a material having a lowfrictional resistance.
 6. A brake pressure proportioning valve accordingto claim 4, wherein the inner diameter of said thrust washer is suchthat the clearance between the inner diameter of said thrust washer andsaid upper flange is minimum so as to prevent a joint portion betweensaid O-ring portion formed at the outer periphery of said sealing memberand said flanges formed at the inner periphery thereof from beingexcessively deflected radially upwardly by the fluid pressure in saidsecond chamber.
 7. A brake pressure proportioning valve according toclaim 6, wherein said housing has a cylindrical bore adjacent saidannular shoulder in said first chamber into which said lower flange ofsaid sealing member extends, the inner diameter of said cylindrical borebeing such that the clearance between the inner diameter of saidcylindrical bore and said lower flange is minimum to prevent said jointportion between said O-ring portion formed at the outer periphery ofsaid sealing membEr and said flanges formed at the inner peripherythereof from being excessively deflected radially downwardly by thefluid pressure in said first chamber.
 8. A brake pressure proportioningvalve according to claim 4, further comprising a back-up ring which isreceived in a cylindrical bore which is larger in diameter than saidcylindrical cavity of said cylindrical extension and which is formed atan end portion of said cylindrical extension of said first end closuremember, the inner diameter of said back-up ring being sized to have avalue to slidably accommodate therein said upper stem portion of saidpressure responsive differential piston with a minimum clearance toprevent the lip portion of said upper flange from being pressed into agap formed between said cylindrical cavity of said cylindrical extensionand said upper stem portion of said pressure responsive differentialpiston thereby to prevent said lip portion from being damaged when saidupper flange of said sealing member is deflected upwardly by a fluidpressure in said second chamber, the axial thickness of said back-upring and the depth of said cylindrical bore being determined to providea minimum clearance between said upper flange and one side of saidback-up ring, and clearance between the inner diameter of saidcylindrical bore and the outer diameter of said back-up ring beingselected to have a sufficient value to permit eccentricity to be causedbetween said upper stem of said pressure responsive differential pistonand said cylindrical extension of said first end closure member.
 9. In ahydraulic brake pressure proportioning valve interposed, in a vehiclehaving a separated hydraulic brake system with front and rear wheelcylinders and a tandem master cylinder having front and rear sectionsrespectively connected through independent fluid circuits with saidfront and rear wheel cylinders, said hydraulic brake pressureproportioning valve comprising; a housing having a first inlet portcommunicating with said front section of said tandem master cylinder, asecond inlet port communicating with said rear section of said tandemmaster cylinder, and an outlet port communicating with said rear wheelcylinder; first and second chambers formed in said housing, said secondchamber providing fluid communication between said second inlet port andsaid outlet port and having incorporated means therein to pass a fluidpressure from said second inlet port to said outlet port until the fluidpressure prevailing in said second inlet port exceeds a predeterminedlevel; a first end closure member screwed into one end of said housingfor closing an open end of said first chamber and including alongitudinal cylindrical extension having formed at its one end anopening, said longitudinal cylindrical extension also having acylindrical cavity provided with a fluid passageway communicating withsaid first inlet port; a second end closure member screwed into anotherend of said housing for closing an open end of said second chamber; apressure responsive differential piston disposed in said first andsecond chambers, said pressure responsive differential piston serving tomodulate the fluid pressure admitted to said second fluid chamber at apredetermined rate when the fluid pressure in said first and secondchambers exceeds said predetermined level under a circumstance in whicha braking fluid circuit is held in its normal condition for transmittinga modulated fluid pressure to said outlet port and serving to modifysaid rate of modulating the fluid pressure in response to a failure inthe front brake fluid pressure circuit for transmitting a fluid pressurewhich is higher than said modulated fluid pressure to said outlet port,said pressure responsive differential piston having an upper stemeffective sectional area exposed to a fluid pressure in said first inletport, a lower stem effective sectional area exposed to a fluid pressurein said second inlet port and a contact effective sectional area exposedto a fluid prEssure in said outlet port; a valve member operativelydisposed in said second chamber of said housing and being cooperativewith a valve head of said pressure responsive differential piston forrestricting the flow of said fluid pressure from said second inlet portto said outlet port when said fluid pressure in said first and secondchambers increases beyond said predetermined level; a compression springdisposed in said second chamber for urging said pressure responsivedifferential piston to a position in which said valve head is unseatedfrom said valve member for transmitting a fluid pressure from saidsecond inlet port to said outlet port at a predetermined condition; anda sealing member operatively disposed in said housing between said firstand second chambers for sealing off said first and second chambers fromeach other thereby to raise the starting point of fluid modulation to ahigher value and to increase the rate of modulating the fluid pressurein response to a failure in the front brake fluid pressure circuit, saidsealing member including an annular O-ring portion which is interposedbetween an annular shoulder formed in said first chamber of said housingand said cylindrical extension of said first end closure member, anupper flange integral with said O-ring portion and having a lip portionangularly inclined angularly upwardly and a lower flange portionintegral with said O-ring portion and having a lip portion angularlyinclined angularly downwardly, said lip portions of said upper and lowerflanges sealingly engaging with said upper stem portion of said pressureresponsive differential piston.